



i ? ^ "'A * ^* - s nap : 



















>/. 








Bureau of Mines Information Circular/1987 



A Review of the Mechanisms 
of Gas Outbursts in Coal 

By David M. Hyman 




UNITED STATES DEPARTMENT OF THE INTERIOR 




Information Circular 9155 

A Review of the Mechanisms 
of Gas Outbursts in Coal 

By David M. Hyman 



- 



UNITED STATES DEPARTMENT OF THE INTERIOR 
Donald Paul Hodel, Secretary 

BUREAU OF MINES 

David S. Brown, Acting Director 







Library of Congress Cataloging in Publication Data: 



Hyman, D. 

A review 


M. (David M.) 

of the mechanisms of gas 


outbursts in 


coal. 




(Information circular; 9155) 










Bibliography 


p.lO-11. 










Supt. of Docs 


no.: I 28.27: 9155. 










1. Coal mines and mining. 2. Gas bursts. I. Title. II. Series: Information circular (United 
States. Bureau of Mines); 9155. 


TN295.U4 


[TN313] 


622 s 


[622 


.8] 


87-600180 



CONTENTS 

Page 

Abstract 1 

Introduction 2 

Coal-gas sorption-desorption methods 3 

Borehole prediction method 6 

Mitigation of outburst events 7 

Summary and conclusions 9 

References 10 

ILLUSTRATIONS 

1« Methane emissions from mining events 3 

2. Comparison of theoretical coal chip desorption 4 

3. Volumes of gas released by outburst events 6 







UNIT 


OF MEASURE ABBREVIATIONS 


USED 


IN THIS 


REPORT 


cm 






centimeter 






m 


meter 


cm 


/g 




cubic centimeter 
gram 


per 




m 3 
min 


cubic meter 
minute 


cm 


/(kg 


'min 2 ) 


cubic centimeter 
kilogram square 
minute 


per 




mm 
m 3 /mt 


millimeter 
cubic meter per 


g 






gram 








metric ton 


h 






hour 






MPa 


megapascal 


kPa 






kilopascal 






pet 


percent 


L 






liter 






s 


second 


L/min 




liter per minute 











A REVIEW OF THE MECHANISMS 
OF GAS OUTBURSTS IN COAL 

By David M. Hyman 1 



ABSTRACT 

Outbursts are sudden and violent releases of gas and coal that result 
from a complex function of geology, stress regime, and gas pressure and 
content. The Bureau of Mines has reviewed methods for prediction and 
mitigation of such outbursts in use worldwide, as an aid in selecting 
the proper techniques for use in specific mine environments. Outburst- 
prone coal may be distinguished from normal coal by its sorption- 
desorption velocity. Three types of methods used to characterize the 
kinetics of sorption-desorption are described; all are based on the 
ability of outburst-prone coal to release, through desorption, methane 
or carbon dioxide much more rapidly than normal coals. Other prediction 
methods, based on borehole samples, are also described. 

Various mitigation methods described and evaluated include (1) working 
the least stressed, less disturbed, lowest gas content seam in multiple- 
seam areas; (2) mine opening geometry; (3) inducer shot firing; (4) 
water infusion; (5) localized stress relief, using boreholes or by cut- 
ting a reliever slot in the longwall face; and (6) other gas drainage 
methods. 



1 Geologist, Pittsburgh Research Center, Bureau of Mines, Pittsburgh, PA. 



INTRODUCTION 



An outburst is defined as a violent, 
simultaneous release of gas(es) and com- 
minuted rock material into a working face 
or the interior of a borehole. In gen- 
eral, an outburst event has the following 
phases (1_) : 2 

1. A stressed volume of rock contain- 
ing gas(es) is exposed to a rapid change 
of confining stress. This rock volume 
has been highly fractured as a result 
either of some preexisting geologic dis- 
turbance (such as a fault) or of mining- 
induced stress concentration. 

2. Gas(es) adsorbed in or contained in 
sandstone or evaporite rocks are rapidly 
released into the fractures, which al- 
ready contain free gas. When more gas 
enters the fracture space than can be 
transported away through the less per- 
meable rock body, a state of stress due 
to gas pressure may be reached where the 
rock body cannot contain the increasingly 
stressed fractured rock volume. 

3. When the rock body can no longer 
contain the stressed and fractured rock 
volume, containment ceases and the frac- 
tured rock mass and gas(es) undergo move- 
ment as they are driven by the gas into a 
pressure sink, e.g., a mine opening or 
borehole. 

4. After the movement of the fractured 
rock and gas(es), there may be continued 
gas flow from the fractured but in-place 
rock that forms the outburst cavity. 
This gas flow generally decreases over 
time. 

Two major theories — the "pocket" and 
the "dynamic" theories — can describe the 
basis of the coal outburst mechanism. The 
pocket theory holds that there exist cer- 
tain volumes of "soft" or crushed coal 
enclosed by "harder" or less fractured 
coal that form reservoirs of gas con- 
tained in the fracture void space. These 
crushed coal volumes are associated with 
faulted or sheared zones and with in- 
tensely folded strata. This comminuted 

2 Underlined numbers in parentheses re- 
fer to items in the list of references at 
the end of this report. 



coal has little unconfined compressive 
strength and is separated from the mine 
opening by an intact zone of coal under 
sufficient stress to become a "permeabil- 
ity dam." When mine development ap- 
proaches a "soft coal" region, an out- 
burst can result if the region is not 
sufficiently drained of free gas and/or 
the stresses in the region are not 
dissipated (2). 

The dynamic theory holds that a volume 
of relatively gassy coal, which is highly 
stressed and penetrated by mining-induced 
fractures, is outburst prone. When a 
mine opening and induced stresses ap- 
proach such a coal volume, the coal frac- 
tures, releasing high-pressure desorbed 
gas, and the coal face fails, resulting 
in an outburst (2)» 

Common to both theories is high-gas- 
content fractured coal that is able to 
desorb gas rapidly upon release of con- 
fining pressure. This rapid desorption 
feature of out burst -prone coal is the ba- 
sis for a rather extensive set of predic- 
tive methods, which are detailed later in 
this report. 

Other aspects of out burst -prone coal 
include low in situ strength due to fis- 
suring, high free-gas pressure, and asso- 
ciation with geologic structures such as 
fracture zones and igneous dikes. These 
aspects are also the basis of a variety 
of predictive methods (2_). 

Outbursts in coal mines represent con- 
siderable hazards. The most immediate 
hazard is the unexpected inundation of 
the ventilation systems with asphyxiating 
volumes of gas. When methane is the re- 
leased gas, an explosive hazard can be 
created, possibly exacerbated by ejected 
coal dust. The force of the released gas 
and displaced material can be sufficient 
not only to disrupt mine ventilation but 
to debilitate stoppings and ground con- 
trol structures such as arches and posts, 
and to injure or kill mine personnel. 
Additionally, an outburst zone presents a 
ground control problem due to the fissile 
nature of the rock that forms the remain- 
ing outburst cavity. Furthermore, gas 
may continue to be emitted, and without 



appropriate ventilation can accumulate in 
the outburst cavity. 

While the scientist and researcher 
would prefer to describe the mechanics of 
coal-gas outburst in very exact quantita- 
tive terms, the mining geologist and 
engineer need to reliably foresee the 
preconditions and precursors. The body 
of literature concerning coal-gas out- 
bursts has abundant works (1-8) that 



represent overviews of the outburst 
phenomena at both national and inter- 
national levels. Case studies of out- 
bursts are extensive, and the bibliog- 
raphies of the aforementioned references 
contain numerous examples. An overview 
of some of the more commonly practiced 
coal-gas outburst prediction and preven- 
tion methods used was compiled as a re- 
sult of Bureau of Mines research. 



COAL-GAS SORPTION-DESORPTION METHODS 



A fundamental component of a coal-gas 
outburst is the ability of coal, whether 
in a fractured or relatively solid state, 
to release sorbed gas fast enough and in 
a large enough volume to overcome con- 
fining stresses and drive the outburst 
process. Whether one subscribes to the 
"pocket" theory of outburst mechanism or 
the "dynamic" theory and its mathematical 
description (9) , the desorption kinetics 
are at the heart of the outburst mechan- 
ism. Studies in West Germany ( 10 ) and 
Wales (11) suggest that outburst-prone 
coal has essentially the same gas content 
and capacity as normal coal. Figure 1 




shows this as well. What distinguishes 
the coals in terms of outburst potential 
is their sorption-desorption velocities. 
A popular theory holds that outburst- 
prone coal is much more extensively mi- 
crofissured than normal (non-outburst- 
prone) coal. This permits a shorter 
diffusion path and a higher surface-to- 
volume ratio. 

To compare the desorption rates between 
outburst-prone and normal coal, a gas 
emission equation and some of its con- 
stants from the literature (2, 11) were 
used in the following analysis. The 
emission equation used is Airey's empiri- 
cal relationship for gas emission from 
coal lumps (12) : 

V(t) = A(l-e( (t/t o )n ) 



= A(l-exp( ( - t/t o )n ) 



and 



where 



A = V 



(k L P) 
L l+k L P 



V(t) = volume of methane desorbed after 
time t, min, 



ROCK MINED OR OUTBURSTED, mt 
FIGURE 1.— Methane emissions from mining events. 



A = 



to = 



n = 



V, = 



ki = 



equilibrium sorption capacity of 
coal at gas pressure, P, kPa, 

time constant min related to coal 
chip size, min, 

constant related to coal type or 
rank, 

maximum Langmuir sorptive capacity 
of coal sample, cm 3 /g, 

Langmuir strength of attraction 
for gas to sorb, kPa -1 . 



The value of t is defined as the time 
required for the subject coal sample of a 
given effective chip size to desorb 63 
pet of its gas content. A calculated t 
range of about 5 to 15 min (11 ) is con- 
sistent with microfissure densities with 
corresponding t values for outburst- 
prone coals (2^ 12). The coal constant, 
n, had been found empirically to be about 
0.5 for anthracite, 0.33 for bituminous 
coals, and 0.25 or less for outburst- 
prone coal. As studies have indicated 
that some outburst-prone coals do not 
have a markedly different gas content 
than that of normal coal in the same 
coalbed, the same in situ equilibrium gas 
content, A, will be used in the analysis. 
Assuming that coal chips have been ob- 
tained from both a normal coal volume and 
an outburst-prone zone, the relative 
(with respect to a constant) desorption 
curves for normal bituminous (n = 0.33, 
to = 60 min), anthracite (n = 0.5, t 
= 60 min), and outburst-prone (n = 0.25, 
to = 15 min) coal chip samples are calcu- 
lated and presented as figure 2. As 
shown in figure 2, the outburst-prone 
coals initially desorb at a faster rate 
than normal coals. The highest contrast 
in desorption rates occurs within ap- 
proximately the first 10 min of desorp- 
tion time. It is apparent from this 



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LU 

Q 

LU 

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KEY 
Outburst-prone coal 
Bituminous coal 
Anthracite 



50 



100 
TIME, min 



150 



200 



pressurized cham- 

the chips from 

degradation over 



FIGURE 2.— Comparison of theoretical coal chip desorption. 



simplistic illustration that in order for 
desorption indices to differentiate be- 
tween normal and outburst-prone coals, 
they must be determined within this short 
span of time. It also follows that ex- 
traordinary care must be exercisec in 
quickly obtaining and preserving (if 
necessary) coal chip samples for these 
predictive index determinations. One 
should also recognize that preserving 
coal chip samples in a 
ber does not prevent 
undergoing structural 
time (_5)« 

Three basic classes of tests character- 
ize sorption-desorption kinetics (_5)« 
The first class comprises volume-desorbed 
methods, or so-called AV methods. One 
such method practiced on a working- 
mine-section scale is the V30 index (1, 
13). The volume of methane emitted 
within the first 30 min after shot firing 
is measured and normalized to the mass of 
coal broken by the shot. This value is 
divided by the desorbable gas content 
(qd), which is determined by a coal chip 
desorption test described later in this 
section. Normal coals have a V30 value 
ranging between 0.10 and 0.17, compared 
with about 0.40 for outburst-prone coals 
and >0.60 for outburst coals. This index 
is used in the Federal Republic of Ger- 
many as part of a hierarchy of tests to 
assess the risk of encountering an 
outburst. 

Another volume-type index is used pri- 
marily in Australia to predict the risk 
of outbursts in advance of mining. This 
test is mainly used for carbon dioxide- 
coal outbursts (fO« The Hargraves AV 
index uses a 4-g sample of drill cuttings 
sized from 0.6 to 1.2 mm. The cuttings 
are obtained from a drill hole and sealed 
in a desorption meter within 1 min of be- 
ing cut by the drill bit. The desorbed 
gas volume is measured from 1 to 6 min 
after drilling. When this value exceeds 
1.2 enr/g, the subject coal is considered 
outburst prone. This threshold value is 
gas specific and colliery specific. The 
use of a slightly modified AV index with 
a shorter observation period was at- 
tempted in Belgium with some success. If 
the volume of gas (V1) desorbed between 
35 and 70 s after drilling is greater 



than 0. 1 cm 3 /g, there may be an outburst 
risk; a Vi value greater than 0.2 cm 3 /g 
indicates a serious outburst risk (]_). 

A rather large class of outburst pre- 
diction indices are the popularly known 
AP indices. These indices are based on 
pressure changes during either desorption 
or sorption tests performed on coal chip 
samples generally within the 0.25- to 
0.50-mm size range. The basic AP index 
is the APq-60 of Soviet origin (1_, _7). 
This is a desorption type of test. 
Originally, different coal chip sizes 
were used for different ranks of coal, 
but the 0.25- to 0.50-mm chip size range 
has become standard through practice. A 
3.5-g coal chip sample is placed into a 
6. 5-cm chamber that has a free gas space 
of 4 era . Other workers have used 3- to 
10-g samples in a chamber with 4 to 10 
era of free gas space. The chamber is 
then evacuated to a negative pressure of 
about 100 kPa for 90 min to degas the 
sample. The chamber is then pressurized 
to about 100 kPa with helium and evacu- 
ated; then the pressure change is moni- 
tored. A resultant pressure rise repre- 
sents the baseline condition for the test 
procedure, as the helium is not sorbed by 
the coal. The sample chamber is then 
evacuated before being pressurized with 
methane at about 100 kPa for 90 min so as 
to saturate the coal sample. After 
saturation, the sample chamber is con- 
nected to an evacuated chamber to reduce 
the pressure in the sample chamber to a 
negative 100-kPa pressure very rapidly. 
The pressure rise is then measured 10 to 
60 s after this chamber pressure reduc- 
tion. The APo- 60 index is equal to the 
pressure rise at 60 s minus the baseline 
pressure rise for the assumed inert (with 
respect to sorption by coal) helium. The 
APio-60 index is the difference between 
pressure rises measured at the 10- and 
60-s time periods and indicates outburst- 
prone conditions when it is greater than 
about 1.3 kPa. When APo- 60 is greater 
than 2 kPa, the sample is considered to 
represent outburst-prone conditions. The 
primary disadvantages of the APo-60 test 
are that it is a laboratory-based deter- 
mination and requires 6 to 8 h to perform 
(1). 



A variation on the APo-60 method, de- 
veloped by Lama (_5 ) , uses shorter obser- 
vation times to evaluate the sorption 
kinetics of coal samples. This method is 
known as the AP express method and cor- 
relates rather well with the APq-60 
method (_5_). For the AP express method, a 
50-g sample of coal chips within the 
0.25- to 0.50-mm size range is sealed in 
a 250-cm chamber. This chamber is 
evacuated for 5 min at a negative pres- 
sure of 100 kPa. After degassing, the 
sample chamber is pressurized with meth- 
ane at about 200 kPa. The pressure drop 
due to adsorption is then measured for a 
10-min period. While the AP eX pr ess index 
has a good correlation with the APo-60 
index, its usefulness as an outburst pre- 
diction has not been demonstrated. If 
the pressure drop curve due to adsorption 
for the APexp ress method is examined for 
the 5- to 10-min time interval, and this 
pressure drop value (expressed in kPa) is 
divided by 300 s, the Li index results. 
This L] index is a measure of sorption 
rate and has had some success in detect- 
ing shear zones of coal. 

A third class of indices for predicting 
outburst-prone coal measures the rate of 
change of desorption rates or desorption 
deceleration. In the Federal Republic of 
Germany a series of calculations is used 
that is based upon the desorption decel- 
eration of borehole cuttings collected 
and sealed in a desorption meter within 
1 min of cutting (1_, J^0« About 10 g of 
coal cuttings in the 0.4- to 0.63-mm size 
range are collected, and the desorption 
rates are measured over a 5- to 10-min 
period. For this time period, a power 
law relationship for desorption rate over 
time is assumed: 

dV(l) = dV(t) ( k v 

dt dt V ' 

where time (t) is in minutes. 

If this power law relationship is 
obeyed, a plot of logarithmic desorption 
rate versus logarithmic time will yield a 
straight line of slope k. The intercept 
at time = 1 min is the desorption rate at 
1 min. When this intercept value is 



multiplied by a time constant, which is a 
function of coal chip size, the absorb- 
able gas content, qd, is obtained. A 
value of qd greater than 9 m /mt indi- 
cates a suspected outburst condition. 
This qd, or desorbable gas content value, 
is the scale for the V30 index described 
earlier. Note that the 9-m /mt threshold 
for outburst-prone coal is very close to 
the smallest specific emission or out- 
burst gas content for coal outburst pre- 
sented in figure 3. A value of k greater 
than 0.75 cm 3 /(kg*min 2 ) indicates that 
the coal sample is from an outburst-prone 
area. Normal coals have a k value of 
about 0.65 cm /(kg'min ). The time con- 



106 



stant, A, used to calculate qd is 
min for the 0.4- to 0. 63-mm coal 
size range and about 25 min for the 
conventional 0.25- to-0.5-mm coal 
size range. 

What these methods, and the 
scribed in the literature have 
is a means to differentiate 
prone coals from normal coals. 



29.4 
chip 
more 
chip 

others de- 

in common 

outburst- 

This is 

based upon the ability of the former to 
release, through desorption, methane 
and/or carbon dioxide much more rapidly 
than normal coals. Such is the basis of 
the British desorption ratio, where a 
sample's desorbable gas content for a 
certain time period is divided into a 
mine-specific representative desorption 
volume. When this ratio exceeds 4, then 
the sample in question is considered to 
represent an outburst-prone zone (.!_)• 
This property of very rapid desorption 
is a fundamental precondition to the 



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N / • Coal outbursts (CH4) 


^/ A Sandstone outburst (CH 4 ) 


X / 


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/ ° Evaporite outburst 

1 1 1 



10' 



I0 6 



I0 2 I0 3 I0 4 I0 5 

ROCK EJECTED, mt 

FIGURE 3. — Volumes of gas released by outburst events. 



development of gas and coal outbursts. 
Given coal chips of a particular size 
range (e.g., 0.25 to 0.50 mm) from 
outburst-prone coal and normal coal, one 
would expect the outburst-prone coal 
chips to have a smaller effective size 
due to some partitioning feature (such as 
microf issuring) in their structure that 
would help explain their faster desorp- 
tion kinetics. One might also expect 
this smaller effective size to contribute 
to increased friability and lower 
strength in comparison to normal coal in 
the same coalbed (10-11). Other types of 
outburst-prediction methods also take 
advantage of these aspects. 



BOREHOLE PREDICTION METHOD 



During the drilling of boreholes into 
outburst-prone zones, drillers often note 
gas "kicks," increased gas flows, and 
disproportionately large volumes of drill 
cuttings (10). In the Federal Republic 
of Germany, a drill-cuttings-to-hole 
volume ratio greater than about 3:1 to 
7:1 is indicative of outburst conditions 
in the coal penetrated by the drill hole. 
The borehole diameters ranged from 50 to 
140 mm in the work reported (10). A 
French study did not establish an out- 
burst risk threshold for drill cutting 
volumes from 43-mm-diameter drill holes 



even though 2 to about 130 times more 
cuttings were encountered than could be 
accounted for by hole volume (J L )» Since 
volume measurements of drill cuttings are 
not accurately reproducible owing to dif- 
ferences in bulking between samples, a 
gravimetric method would be more useful. 

A more direct and hopefully more useful 
prediction method based on the sheared 
and/or low-strength qualities of 
outburst-prone coal is presented by 
Kidybinski (15-16), whose two papers de- 
scribe the use of a borehole penetrometer 
with a conical tip to determine faulted 



areas and adjacent zones of relatively 
soft coal, as an outburst prediction 
tool. This use of this tool yields a 
coal and/or rock strength index, Z, that 
is equal to the applied thrust on the 
cone divided by the penetration distance. 



In tests at three mines, the coal 
strength was found to be related to in 
situ gas pressures. Some preliminary 
empirical relationships were developed, 
but more research is required to develop 
a more definitive relationship. 



MITIGATION OF OUTBURST EVENTS 



The ultimate method for preventing an 
outburst event during mining is to pre- 
dict potential zones of outbursts and 
avoid them or at least reduce their out- 
burst potential. As the outburst mechan- 
ism is a complex relationship between 
geologic structure, mining-induced stres- 
ses, and gas content and pressure, the 
removal or mitigation of one or more of 
these elements can possibly reduce out- 
burst potential. Geologic structures 
such as faults or sheared zones can be 
avoided to a certain extent. Stresses 
can be reduced by changes in mining 
rates, methods, and geometry. Gas con- 
tents and pressures can be reduced by 
drainage. Most of the methods practiced 
throughout the world involve relieving 
stress concentrations and/or in situ gas 
pressure. 

A whole-seam stress reduction method 
applicable in areas where several minable 
coal seams occur in close proximity is 
known as "working the protective seam" 
(1_). The idea behind this method is to 
mine the least stressed, and/or lowest 
gas content, and/or least disturbed coal- 
bed of those in a multiple-seam configur- 
ation. It is preferred to mine as a pro- 
tective seam an overlying one, instead of 
an underlying one, unless the outburst 
problems are more severe than the ground 
control problems due to subsidence from 
undermining. If conditions permit, the 
pillars could be superimposed for the 
protective and outburst-prone seams so as 
not to defeat the stress-relief aspect of 
this protective method. Mining the pro- 
tective seam can also induce some fissur- 
ing and thus potentially drain some por- 
tion of the gas in the outburst-prone 
seam. Not only does the gas drainage ef- 
fect tend to help lower the overall gas 
content of the outburst-prone seam, it 
also can help lessen the magnitude of 
the desorption rate by allowing the 



desorption process to begin prior to min- 
ing. The overall effect of mining a pro- 
tective seam is to reduce the stress and 
gas dynamic potential fields of the 
outburst-prone seam (17). Thus, it is no 
surprise that this method is one of the 
most effective methods of reducing out- 
bursting probability. Unfortunately, 
this method is applicable only in multi- 
seam configurations. In single-seam con- 
figurations, other methods must be 
employed to reach the same end of reduc- 
ing the stress and gas dynamic potential 
fields of an outburst-prone coalbed. 

Given an outburst-prone coalbed, a 
variety of methods have been practiced to 
relieve stress concentrations. Mine 
opening geometry control is a relatively 
effective method. Stress concentrations 
are greater at the face of a single 
heading or roadway than along a longwall 
face. Retreat longwalls are less prone 
to outbursts than advancing longwalls. 
Pillar extraction is less prone to out- 
bursting than retreating longwalls, but 
this may be due to degassing more than to 
stress relief. The most outburst-prone 
mining operation is when a heading or 
tunnel moves from one coal seam through a 
rock interburden to an adjacent coal seam 
(4)» In general, longwall mining methods 
with gate roads not developed more than 2 
m ahead of an advancing longwall face are 
less liable to trigger outbursts than are 
room-and-pillar mining methods (_1_)» 

Besides mine opening geometry, several 
stress-relief measures are practiced at 
the mine face level. Inducer shot firing 
is employed in several countries to 
relieve stress accumulations and to trig- 
ger outbursts in a relatively controlled 
fashion. One form that inducer shot fir- 
ing takes is destressing at the ends or 
gate road faces of a longwall panel. 
This precautionary shot firing involves 
drilling holes outby the gate road faces, 



usually to a depth of about 3.7 to 4.6 m, 
although in Turkey holes up to 8.3 m deep 
are used (8). These holes are charged 
with explosives. When an outburst-prone 
zone is predicted within the longwall 
face area, shot firing is performed by 
detonating explosives in boreholes across 
the longwall face on both sides of the 
suspected zone (I) . The explosive 
charges can be detonated either simul- 
taneously (camouflet) for maximum shock 
loading, or with short delays to facili- 
tate mucking operations. A variation on 
this shot-firing theme is termed pulsed 
infusion shot firing. For this method 
boreholes 4 to 9m deep are drilled, 
charged with submarine explosives, filled 
and pressurized with water, and deto- 
nated. Although shot firing has been 
used with some success in controlling the 
occurrence of outbursts and in mitigating 
to some extent the severity of further 
outbursts in the treated area, it is an 
inherently hazardous practice; also the 
deleterious effects of expelled coal and 
gas due to outbursting still occur and 
must be considered when using these 
techniques. To a limited extent, the 
erection of a barricade 8 to 15 m from 
the face can mitigate these deleterious 
effects. Unfortunately, such barricades 
introduce problems for postshot face 
ventilation gas checks (4^« 

A less energetic method of destressing 
and fracturing an outburst-prone zone in 
a coalbed is through water infusion. 
Water infusion for destressing is a modi- 
fied form of water infusion for dust and 
gas emission control. It is performed 
where prediction methods such as prox- 
imity to a fault, high Po-6 values, or 
discharge of a large relative volume of 
drill cuttings indicate an outburst-prone 
zone. The infusion method has been 
implemented in the Federal Republic of 
Germany (19) by drilling 50-mm-diameter 
boreholes at a spacing of three to four 
gate road widths in the suspect zone. 
The boreholes are drilled into the sus- 
pect zone and generally are less than 10 
m from the face; depending on the nature 
and extent of the suspected outburst- 
prone zone, they can be up to 60 m from 
the face. Water is pumped into the holes 
at about 70 to 100 L/min, at pressures up 



to 40 MPa but averaging 11 to 23 MPa. 
Each hole pattern is infused sequentially 
until either cracking or separation of 
the coalbed occurs. The infused water 
both redistributes stress and forces free 
gas in the fractures away from the face. 
Water infusion becomes hydraulic fractur- 
ing when a series of pressure pulses are 
applied to the water in the hole. A 
major drawback to water infusion is that 
its degassing or gas-displacing effects 
are relatively short-lived; consequently, 
it must be performed frequently, poten- 
tially interfering with production. 

Localized stress relief can be accom- 
plished through the use of boreholes in 
terms of relieving both local stress and 
gas pressure. These stress-relief bore- 
holes have diameters of about 82 to 300 
mm. An effective borehole diameter is 
found by drilling holes of progressively 
larger diameters until an excessive vo- 
lume of cuttings is discharged, indicat- 
ing a miniature outburst event in the 
hole. The relief hole spacing is found 
by reducing borehole spacing until no 
audible stress readjustments are heard 
(4) as the holes are bored. The length 
of these boreholes is 10 to 25 m in ad- 
vance of the face. Additionally, cavi- 
ties can be excavated at some depth in 
the borehole to trigger an outburst in 
the borehole to further relieve stress 
and gas pressure. This practice is known 
as "perforation" in Hungary and has also 
been used in the U.S.S.R. and Australia. 
A considerable drawback to perforation is 
controlling the coal dust and gas ejected 
from the hole. 

A stress-relief technique practiced in 
the U.S.S.R. is the cutting of a reliever 
slot in the longwall face. A cutting 
cable saw is used to cut an 80-mm slot 
about 3 to 5 m deep, effectively under- 
cutting the face. Different slot orien- 
tations and cutting depths are used, 
depending on local conditions (17). 

Relief of excess stress removes, 
through redistribution, a portion of the 
triggering energy for an outburst. The 
reduction of gas pressure as well as gas 
content also removes potential energy 
from an outburst-prone zone. An 
outburst-mitigation method therefore re- 
duces gas pressure in a gas drainage 



borehole, as well as reducing stress con- 
centrations to a limited extent. 

In general, gas drainage boreholes have 
smaller diameters (40 to 100 mm) than 
stress-relief boreholes. The strategy in 
gas drainage is to degas volumes of rock 
in advance of mining with lateral and 
vertical holes in gate roads and faces 
(4_). It has been found that open hole or 
free-flow drainage is not as effective or 
efficient as applying a negative pressure 
of 7 to 40 kPa. Studies (1_, IT, 1_9) have 
shown that application of a slight nega- 
tive pressure to increase the borehole 
pressure sink increased gas output from 
the drainage holes by factors of 2 to 4 
times compared with free-flow drainage. 
What determines the spacing of boreholes 
in a gas drainage plan is a complex func- 
tion of coalbed permeability, gas pres- 
sure, pressure gradient, degree of water 
saturation, gas dynamics of the coal, and 
duration of drainage. A Chinese experi- 
ment (1_) involved the drilling of 22 gas 
drainage holes of 75-mm diam into an 
outburst-prone coalbed. These holes were 
monitored for 19 months. When mining 
operations penetrated the drained zone, 
no outburst events occurred. A 6- to 7-m 
radius of influence was calculated, 
indicating that a 10- to 15-m gas drain- 
age hole spacing would be effective for 
this mine. The Japanese (20) performed a 
gas drainage study in which 65- to 90-mm 
boreholes were drilled at 10- to 20-m 
spacings. The gas drainage was aided by 
suction. During drainage, test holes 
were bored and gas pressure and flow mea- 
surements were made. Mining was started 
when the test boreholes showed that gas 
pressures and flows had been lowered. 
This gas reduction occurred after 1 to 2 
months. 

SUMMARY AND 



A gas drainage experiment was conducted 
in an Australian coalbed containing shear 
zones that were outburst prone (2_0. The 
experiment consisted of drilling three 
100-mm-diameter holes parallel to each 
other and separated by about 10 m. The 
holes were drilled to 23-, 45-, and about 
60-m depths. The longest hole penetrated 
a shear zone. Gas pressure and flows 
were monitored for about 140 days. The 
shortest hole produced 0.35 L/min per 
meter of length, the 45-m hole produced 
about 2 L/min per meter, and the hole 
that penetrated the shear zone produced 
63 L/min per meter. The gas pressure in 
the shear zone dropped from an initial 
410 kPa to about 200 kPa after 30 days 
and to about 100 kPa after 80 days. The 
depth of the coalbed was about 500 m, and 
the holes were drained without the ap- 
plication of suction. When mining inter- 
cepted this drained shear zone, no out- 
bursts occurred. In another experiment 
in the same mine, 40-kPa suction was ap- 
plied to boreholes, yielding 200- to 400- 
pct increases in flow rates. The range 
of influence for drainage holes in this 
mine was estimated to be about 30 m. 

While not all gas drainage exercises 
reported in the literature have prevented 
outbursts, they have been somewhat ef- 
fective in draining gas and reducing 
gas pressure in advance of mining. 
Gas drainage works best in outburst 
zones where the coal is crushed or 
comminuted, as in a fault or shear 
zone. Outburst-prone zones where the 
coal is relatively intact are not very 
amenable to gas drainage without the 
application of borehole stimulation 
techniques. 



CONCLUSIONS 



The body of scientific literature that 
describes outburst mechanics, precondi- 
tions, prediction techniques, and pre- 
vention or defensive measures is exten- 
sive. Effective management of outburst 
zones requires that they be predicted in 
advances of mining by geological in- 
vestigations to delineate fault or shear 
zones and igneous intrusions. A variety 



of physical testing methods is available 
to predict outbursts, centering on rela- 
tively rapid (on the order of minutes) 
desorption tests. Borehole prediction 
methods rely on the instability and fis- 
sile nature of outburst-prone coal or gas 
and comminuted coal expulsions from bore- 
holes. As outbursts are a complex func- 
tion of geology, stress regimes, and 



10 



gas contents, available defenses attempt 
to relieve stress and gas pressures once 



an outburst-prone zone has 
and geologically mapped. 



been defined 



REFERENCES 



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Nedra Press, 1981, 335 pp. 



11 



18. Grobkrenz , H. De-stressing Infu- 
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